The invention pertains to methods and apparatus for transferring, by projection, a pattern defined by a reticle or mask onto a sensitized substrate using a mirror-projection system, such as an X-ray optical system.
A projection-exposure apparatus for fabrication of integrated circuits projects and transfers a circuit pattern defined by a reticle or mask (these terms are used interchangeably herein) onto a sensitized substrate (e.g., a semiconductor wafer) through an image forming-apparatus. Such exposure apparatus conventionally use an illumination source (light source), such as an i-line light source. Upon illumination of the reticle with the light source, the illuminated pattern defined by the reticle is projected and inscribed on the substrate. The pattern defined by the reticle is typically either larger than or equal in size to the pattern to be inscribed on the substrate.
Conventional exposure apparatus typically include an illumination-optical system and a projection-optical system. A substrate stage adjusts the substrate""s axial position (height) and inclination. The substrate stage also adjusts the substrate""s position on the XY plane. When projection-exposing a reticle pattern onto a substrate, the patterned surface of the reticle is first aligned with the substrate surface; that is, the focal points are aligned. For projection exposure using light (e.g., UV light), the projection-optical system ordinarily comprises a plurality of lenses and is arranged so that the pattern formed on the reticle can be focused on the surface of the substrate and transferred in a single shot. Since most projection-optical systems have a field of view of approximately 20 mm2, a desired region (e. g., typically a region equivalent to two semiconductor chips) can be exposed at one time.
In recent years, as progress has been made in terms of higher performance and a higher degree of integration in semiconductor integrated circuits, there has been continual demand for increased accuracy and resolution in pattern transfer. Accordingly, errors with respect to imageformation characteristics that accompany inaccurate measurement of the substrate surface height (often caused by insufficient flatness of the substrate surface and/or substrate stage) cannot be ignored.
A focal-point (axial-position) detection system is typically used to determine the substrate surface height in order to achieve alignment of the focal point across a wide exposure field. A slit image is first projected obliquely relative to the optical axis AX (i.e., the slit image is not projected through the projection-optical system) onto each of multiple measurement points located inside the shot region (exposure field) of a substrate. The axial-position detection system (based on the oblique-incidence method) is used to receive light from the reflected image by a two-dimensional array sensor.
Generally, the resolution W of an exposure apparatus is determined by the exposure wavelength xcex and the numerical aperture NA of the projection-optical system. The resolution W is calculated as follows:
W=k1xcex/NA
wherein k1is a constant. In order to increase the resolution, it is necessary to shorten the wavelength of the illumination source or to increase the numerical aperture. For example, when an i-line light source having a wavelength of 365 nm is used for illumination, a resolution of 0.5 xcexcm is obtained at a numerical aperture of approximately 0.5. It is difficult, however, to increase the numerical aperture in such a system. Accordingly, it has been necessary to further shorten the wavelength of the illumination source.
Excimer lasers have begun to be used as sources for illumination as excimer lasers produce light with wavelengths that are shorter than i-line (e.g., 248 nm for KrF and 193 nm for ArF excimer lasers). A resolution of 0.25 xcexcm may be obtained when using a KrF illumination source, and a resolution of 0.18 xcexcm may be obtained when using an ArF exposure-illumination source. If X-rays (having a wavelength of about 13 nm) are used as the illumination source, a resolution of 0.1 xcexcm or better may be obtained.
When such exposure apparatus use X-rays as the illumination source, the projection-optical system must be constructed entirely from reflective mirrors. Unfortunately, it is difficult to design such a projectionoptical system having a broad exposure field. In addition, when attempting to design such a projection-optical system having a higher resolution, the exposure field is even further reduced. A smaller exposure field has the necessary result that the desired pattern region cannot be exposed in a single shot. Accordingly, integrated circuit throughput is decreased and manufacturing costs are increased.
If the exposure field of the projection-optical system is formed in the shape of an annular band, a higher resolution may be obtained in a long, narrow exposure field. Semiconductor chips that are 20 mm2 or larger may be exposed, even with a projection-optical system having a small exposure field, by scanning the reticle and substrate during the exposure process.
A conventional X-ray-projection-exposure apparatus is illustrated in FIG. 6. The X-ray-projection-exposure apparatus shown in FIG. 6 includes an X-ray source 61, an illumination-optical system 62, a projection-optical system 51, a reticle stage 53 that secures a reticle 52, and a substrate stage 55 to which a substrate 54 is mounted. A vacuum chamber 56 encloses the X-ray source 61, illumination-optical system 62, projection-optical system 51, reticle 52, reticle stage 53, substrate 54, and substrate stage 55. The exposure apparatus further includes a axial-position detection system 57a, 57b. The axial-position detection system 57a, 57b is positioned outside the vacuum chamber 56.
The illumination-optical system directs X-rays to irradiate a portion of the pattern defined by the reticle. The projection-optical system 51, typically comprising a plurality of mirrors, is arranged so that the pattern on the reticle 52 is reduced, projected, and focused onto the surface of the substrate 54. Multi-layer films are formed on the surfaces of the projection-optical system 51 reflective mirrors to heighten the X-ray reflectivity. The projection-optical system 51 has an annular-band shaped exposure field. The projection-optical system 51 projects a portion of the reticle pattern in an annular-band shape onto the surface of the substrate 54. During exposure, the reticle 52 and substrate 54 are synchronously scanned at a constant speed, so that the desired pattern region on the reticle (e. g., a region corresponding to a single semiconductor chip) can be projected onto the substrate 54.
Soft X-rays having a wavelength of approximately 13 nm, for which a high reflectivity is obtained from the multi-layer films of the projection-optical system 51, may be used as the illumination source. However, such soft X-rays are extensively absorbed by air. Accordingly, at least the reticle 52, substrate 54 and projection-optical system 51 must be disposed inside the vacuum chamber 56 so that the light path of the X-rays is maintained in a vacuum. The interior of the vacuum chamber 56 is evacuated by means of a vacuum pump (not shown).
In such an X-ray-projection-exposure apparatus, since the substrate 54 must be disposed in the vacuum chamber, at least a portion of the light beam of the axial-position detection system 57a, 57b passes through the vacuum. However, a conventional axial-position detection system cannot be disposed in a vacuum. If the entire axial-position detection system is disposed in a vacuum, radiation of heat generated by the axial-position detection system light source (e.g., a halogen lamp) becomes a problem. Conventionally, when such a light source is operated in an air atmosphere, heat generated by the lamp is dissipated into the surrounding atmosphere. However, if such a light source is operated within a vacuum, the significant amount of heat generated by the light source cannot be dissipated. As a result, the heat melts the light source. Accordingly, conventional X-ray-projection-exposure apparatus include a axial-position detection system wherein both the irradiating device 57a and the detection device 57b are disposed outside the vacuum chamber 56.
As shown in FIG. 6, first and second windows 60a and 60b (through which the axial-position detection light beam passes) are formed in the vacuum chamber 56. The first window 60a is disposed between the irradiating device 57a and the substrate 54. The second window 60b is disposed between the detection device 57b and the substrate 54. A light beam 58a passes through the first window 60a and strikes the substrate 54. The light beam 58b reflected from the surface of the substrate then passes through the second window 60b and is conducted to the detection device 57b. 
When the axial-position detection system is constructed as described above, however, even though the height of the substrate 54 can be measured, the focal-point-detection device cannot be disposed in the X-ray-projection-optical system 51. As a result, the relative distance between the projection-optical system 51 and the substrate 54 cannot be accurately determined. Accordingly, there is a drop in reproducibility of the axial-position detection measurement leading to lower precision integrated circuit formation.
Accordingly, there is a need for an X-ray projection-exposure apparatus that is capable of performing high-accuracy detection of axial position without causing untimely deterioration of the axial-position detection optical components.
In view of the shortcomings of the prior art, the present invention provides, inter alia, X-ray-projection-exposure apparatus comprising an X-ray source and an X-ray illumination-optical system for directing X-ray light to irradiate a pattern defined by a reticle. The exposure apparatus further includes a projection-optical system for directing illumination light along a Z axis from the reticle pattern to project an image of the reticle pattern on a sensitive surface of a substrate (the Z axis being an optical axis of the projection-optical system). A substrate stage is operable to hold the substrate for exposure of the sensitive surface and to move the substrate in the X, Y, and Z directions.
The exposure apparatus further includes a reticle stage that secures the reticle such that the pattern on the reticle may be irradiated by the illumination-optical system. A vacuum chamber encloses the projection-optical system, the reticle stage and reticle, and the substrate stage and the substrate. The vacuum chamber further encloses an axial-position detection system (e.g., focal-point detection system). The axial-position detection system operates to optically detect the position of the substrate surface (thus, the focal point) in the direction of the optical axis of the projection-optical system.
The axial-position detection system of the exposure apparatus of the present invention preferably comprise a first irradiating assembly and a second irradiating assembly. The first irradiating assembly includes a light source positioned and configured to illuminate a first slit. The first irradiating assembly also includes at least part of a light-transmitting system situated and configured to conduct light from the light source to the first slit. The second irradiating assembly also includes at least part of the light-transmitting system (not included in the first irradiating assembly) for illuminating the first slit, a first optical assembly for projecting light passing through the first slit to form an image of the first slit on the substrate surface, and a second slit. The axial-point detection system further includes a light-receiving assembly including a second optical assembly for projecting an image of the first slit formed on and reflected from the surface of the substrate onto a second slit. The light-receiving assembly further includes a detector that receives and measures the intensity of light that passes through the second slit. In one embodiment, the vacuum chamber also encloses the first slit, the first and second optical assemblies, and the second slit.
In the X-ray projection-exposure apparatus of the present invention, the principal components of the axial-position detection system may be secured to a projection-optical box (POB) of the projection-optical system. As a result, the substrate height is measured with high accuracy, and the substrate-surface position may be adjusted to a position in the range of the focal depth of the X-ray projection-optical system. Accordingly, a resist pattern having the desired pattern resolution is formed on the substrate surface while maintaining a relatively high throughput in the fabrication of integrated circuits.